Evidence of a dissolution effect on benthic foraminiferal shell chemistry: δ13C, Cd/Ca, Ba/Ca, and Sr/Ca results from the Ontong Java Plateau

Authors

  • Daniel C. McCorkle,

  • Pamela A. Martin,

  • David W. Lea,

  • Gary P. Klinkhammer


Abstract

Core-top benthic foraminifera (Cibicidoides wuellerstorfi) from a depth transect of Soutar box cores from the Ontong-Java Plateau (1.6–4.4 km) were analyzed for cadmium, barium, and strontium (Cd/Ca, Ba/Ca, and Sr/Ca) and for their stable isotopic composition (δ13C and δ18O). We also measured bottom water δ13C, Cd, and Ba at these sites. Foraminiferal δ13C values remain roughly constant over the entire depth range while bottom water δ13C values increase slightly, such that the δ13C difference between C. wuellerstorfi and bottom water ranges from about +0.2 ‰ in cores above 2.5 km to about −0.2 ‰ in cores below 4 km. This apparent depth dependence has not been previously reported, but this range in Δδ13C values is comparable to the uncertainty in published δ13C calibration studies. We observe strong decreases in foraminiferal Cd/Ca, Ba/Ca, and Sr/Ca ratios (50, 25, and 15 percent, respectively) at water depths greater than about 2.5 km. These decreases are substantially larger than the corresponding changes in bottom water trace element concentrations, and they are not correlated with variations in pore water Cd and Ba concentrations at these sites. Together, the foraminiferal and bottom water Cd/Ca, Ba/Ca, and Sr/Ca data yield decreases in the apparent distribution coefficients for these metals into calcite with increasing water depth, again a pattern which has not been previously reported. These results when combined with the data from published core-top calibration studies suggest that a preferential loss of Cd, Ba, and Sr occurs during the dissolution of benthic foraminiferal calcite on the sea floor and raise the possibility of a dissolution-driven decrease in benthic foraminiferal δ13C values.

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